This error condition will sometimes occur when generating reports over a large date-range for sites that see a large amount of traffic. This is due to the archiving mechanism that is used on data that is more than 2 months old. When requesting older data, it takes extra time for Urchin to retrieve and prepare it for reporting. Once prepared, the data is retained for a short while (cached) so that subsequent requests will be much faster.
The workaround is simple: refreshing the page in your browser will instruct Urchin to continue processing where it left off, and will usually provide enough time to finish generating larger reports. If the report still fails to complete before the timeout, it may be necessary to shorten the date range of your requested report.